The long-term goals of the proposed research are both to elucidate central molecular controls over development and diversity of neocortical callosal projection neurons (CPN), and to identify potential causes and therapeutic approaches to disease involving CPN. CPN are the broad population of interhemispheric pyramidal neurons whose axons connect the two cerebral hemispheres via the corpus callosum. CPN play key roles in high-level associative, integrative, cognitive, behavioral, sensory, and motor functions, based on precise, area-specific CPN subtype connectivity and diversity. Disruptions in CPN development are correlated with deficits in multiple cognitive and behavioral disorders, including agenesis of the corpus callosum, autism spectrum disorders, and schizophrenia. Currently, how the remarkable diversity of CPN subtypes and connectivity is specified, and how they differentiate to form highly precise and specific circuits, is unknown. Our lab recently identifieda combinatorially-expressed set of genes that both define CPN as a broad population, and identify novel subpopulations of CPN during embryonic and early postnatal development (Arlotta, Neuron, 2005; Molyneaux, J Neurosci, 2009). Building on our previous work and substantial preliminary data, we propose deep and rigorous functional investigation of Cited2, one newly identified transcriptional co-regulator that we hypothesize functions importantly in development of areally- and functionally-specific subtypes of CPN, notably somatosensory CPN, and its dysfunction might contribute to disorders of CPN connectivity and diversity. We have completed quite substantial, highly motivating preliminary studies of Cited2 function in neocortical development, leading to the hypothesis that Cited2 regulates and refines two stages of precise CPN development and diversity, functioning 1) broadly in basal progenitors to regulate generation of superficial layer CPN, and 2) in an areally-restricted manner to refine distinct identity and development of somatosensory (S1) CPN. To further investigate this hypothesis, we propose to: Aim 1) investigate development and precision of Cited2-null CPN axonal targeting; Aim 2) identify potential postmitotic-specific function(s) of Cited2 that are distinct from Cited2 function in progenitors; Aim 3) investigate CPN-autonomy of Cited2 function during S1 CPN development and connectivity, via novel mosaic, recombinase-based genetic manipulation technology (BEAM) for dual population analysis; and Aim 4) investigate growth cone RNA and proteomes of WT vs Cited2 cKO S1 CPN during axonal development via an entirely new and innovative approach, to gain direct mechanistic understanding of CPN circuit development. Together, these proposed studies will provide substantial insight into molecular controls over development and diversity of CPN subtypes with distinct connectivity, function, and integration of cortical information, processes centrally disrupted in multiple human disorders. Such controls over CPN diversity are currently essentially unknown. This research will contribute to understanding of cortical organization, function, evolution, and potentially toward prevention, diagnosis, and future therapy of human developmental disorders.